As i said, without changing other rules this would be very bad for any movement encounter with a normal (rats&catchers) crew.
You cant move the rats out of Hamelins 6" without risking the swarm to be wiped out immediately. No activating, no new catcher, rats gone.
And it defines the activation order. Activating the catcher first (for example when he needs healing) stops the swarm for the turn if you use Slaughter rats. Sure you can move 4" per turn and lure them with cheese. Enemies will be very happy about a speed of 4-5" for the swarm.
So you always have to activate rats before or the swarm moves unbelievable slow.
This doesnt work with a simple "elegant" change, you would need a full rewrite of the rat/catcher mechanics.